Hitherto, there is a sheet finisher which is placed downstream of an image forming apparatus, such as a copier, a printer or an MFP (Multi-Function Peripheral), and performs finishing, such as punching or stitching, on a printed sheet.
A recent sheet finisher has multiple functions, and a sheet folding apparatus has appeared, which has, in addition to punching and stitching functions, a folding function to fold a part of a sheet, and a saddle-stitching and folding function to staple the center of a sheet and then to fold the sheet at the center.
In the sheet finisher having the saddle-stitching and folding function, it becomes possible to form a booklet (to bind a book) from a plurality of printed sheets.
In the saddle-stitching and folding, after the center of sheets is stitched with staples or the like, a fold line is formed on the stitched part by a pair of rollers called fold rollers. A plate-like member called a fold blade is applied to the stitched part of the sheet bundle, and presses into a nip of the fold roller pair to form the fold line on the sheet bundle.
However, when the number of sheets is large, or the sheet bundle includes a thick sheet, an excellent fold line is not necessarily formed. To cope with this issue, JP-A 2003-182928 discloses a technique d in which a roller called a reinforce roller is additionally provided, and the fold line formed by the fold rollers is reinforced by the reinforce roller.
The reinforce roller moves along the fold line of the sheet bundle pressed out from the fold roller, and reinforces the fold line by the pressure applied to the fold line by the reinforce roller.
As the pressure of the reinforce roller becomes large, a more excellent fold line can be formed. Especially, when the number of sheets is large and the sheet bundle is thick, a large pressure is required.
On reinforcing the fold line, the reinforce roller moves from the outside area of the sheet bundle to the sheet bundle so as to climb over the sheet bundle at the edge of the sheet bundle, and then moves along the fold line of the sheet bundle.
When the reinforce roller climbs over the edge of the sheet bundle, and when the pressure of the reinforce roller is excessively high, there occurs an undesirable phenomenon that the reinforce roller stops at the edge of the sheet bundle, or damages the edge of the sheet bundle. Besides, when the pressure of the reinforce roller is high, an un-neglectable impact sound can occur when the reinforce roller climbs over the edge of the sheet bundle or drops to the outside area of the sheet bundle. These undesirable phenomenon occurred at the edge of the sheet bundle as stated above becomes serious as the sheet bundle becomes thick or the pressure of the reinforce roller becomes high. This contradicts the request that in order to obtain an excellent fold line, as the sheet bundle becomes thick, the pressure of the reinforce roller must be made high.